[Xen-devel] [PATCH v4 1/2] drm: Use srcu to protect drm_device.unplugged

[Oleksandr Andrushchenko <andr2000@xxxxxxxxx>]

From: Noralf Trønnes <noralf@xxxxxxxxxxx>

Use srcu to protect drm_device.unplugged in a race free manner.
Drivers can use drm_dev_enter()/drm_dev_exit() to protect and mark
sections preventing access to device resources that are not available
after the device is gone.

Suggested-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
Signed-off-by: Noralf Trønnes <noralf@xxxxxxxxxxx>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
Tested-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
Cc: intel-gfx@xxxxxxxxxxxxxxxxxxxxx
---
 drivers/gpu/drm/drm_drv.c | 54 ++++++++++++++++++++++++++++++++++++++++++-----
 include/drm/drm_device.h  |  9 +++++++-
 include/drm/drm_drv.h     | 15 +++++++++----
 3 files changed, 68 insertions(+), 10 deletions(-)

diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
index a1b9338736e3..32a83b41ab61 100644
--- a/drivers/gpu/drm/drm_drv.c
+++ b/drivers/gpu/drm/drm_drv.c
@@ -32,6 +32,7 @@
 #include <linux/moduleparam.h>
 #include <linux/mount.h>
 #include <linux/slab.h>
+#include <linux/srcu.h>
 
 #include <drm/drm_drv.h>
 #include <drm/drmP.h>
@@ -75,6 +76,8 @@ static bool drm_core_init_complete = false;
 
 static struct dentry *drm_debugfs_root;
 
+DEFINE_STATIC_SRCU(drm_unplug_srcu);
+
 /*
  * DRM Minors
  * A DRM device can provide several char-dev interfaces on the DRM-Major. Each
@@ -318,18 +321,51 @@ void drm_put_dev(struct drm_device *dev)
 }
 EXPORT_SYMBOL(drm_put_dev);
 
-static void drm_device_set_unplugged(struct drm_device *dev)
+/**
+ * drm_dev_enter - Enter device critical section
+ * @dev: DRM device
+ * @idx: Pointer to index that will be passed to the matching drm_dev_exit()
+ *
+ * This function marks and protects the beginning of a section that should not
+ * be entered after the device has been unplugged. The section end is marked
+ * with drm_dev_exit(). Calls to this function can be nested.
+ *
+ * Returns:
+ * True if it is OK to enter the section, false otherwise.
+ */
+bool drm_dev_enter(struct drm_device *dev, int *idx)
+{
+       *idx = srcu_read_lock(&drm_unplug_srcu);
+
+       if (dev->unplugged) {
+               srcu_read_unlock(&drm_unplug_srcu, *idx);
+               return false;
+       }
+
+       return true;
+}
+EXPORT_SYMBOL(drm_dev_enter);
+
+/**
+ * drm_dev_exit - Exit device critical section
+ * @idx: index returned from drm_dev_enter()
+ *
+ * This function marks the end of a section that should not be entered after
+ * the device has been unplugged.
+ */
+void drm_dev_exit(int idx)
 {
-       smp_wmb();
-       atomic_set(&dev->unplugged, 1);
+       srcu_read_unlock(&drm_unplug_srcu, idx);
 }
+EXPORT_SYMBOL(drm_dev_exit);
 
 /**
  * drm_dev_unplug - unplug a DRM device
  * @dev: DRM device
  *
  * This unplugs a hotpluggable DRM device, which makes it inaccessible to
- * userspace operations. Entry-points can use drm_dev_is_unplugged(). This
+ * userspace operations. Entry-points can use drm_dev_enter() and
+ * drm_dev_exit() to protect device resources in a race free manner. This
  * essentially unregisters the device like drm_dev_unregister(), but can be
  * called while there are still open users of @dev.
  */
@@ -338,10 +374,18 @@ void drm_dev_unplug(struct drm_device *dev)
        drm_dev_unregister(dev);
 
        mutex_lock(&drm_global_mutex);
-       drm_device_set_unplugged(dev);
        if (dev->open_count == 0)
                drm_dev_put(dev);
        mutex_unlock(&drm_global_mutex);
+
+       /*
+        * After synchronizing any critical read section is guaranteed to see
+        * the new value of ->unplugged, and any critical section which might
+        * still have seen the old value of ->unplugged is guaranteed to have
+        * finished.
+        */
+       dev->unplugged = true;
+       synchronize_srcu(&drm_unplug_srcu);
 }
 EXPORT_SYMBOL(drm_dev_unplug);
 
diff --git a/include/drm/drm_device.h b/include/drm/drm_device.h
index 7c4fa32f3fc6..3a0eac2885b7 100644
--- a/include/drm/drm_device.h
+++ b/include/drm/drm_device.h
@@ -46,7 +46,14 @@ struct drm_device {
        /* currently active master for this device. Protected by master_mutex */
        struct drm_master *master;
 
-       atomic_t unplugged;                     /**< Flag whether dev is dead */
+       /**
+        * @unplugged:
+        *
+        * Flag to tell if the device has been unplugged.
+        * See drm_dev_enter() and drm_dev_is_unplugged().
+        */
+       bool unplugged;
+
        struct inode *anon_inode;               /**< inode for private 
address-space */
        char *unique;                           /**< unique name of the device 
*/
        /*@} */
diff --git a/include/drm/drm_drv.h b/include/drm/drm_drv.h
index d23dcdd1bd95..7e545f5f94d3 100644
--- a/include/drm/drm_drv.h
+++ b/include/drm/drm_drv.h
@@ -624,6 +624,8 @@ void drm_dev_get(struct drm_device *dev);
 void drm_dev_put(struct drm_device *dev);
 void drm_dev_unref(struct drm_device *dev);
 void drm_put_dev(struct drm_device *dev);
+bool drm_dev_enter(struct drm_device *dev, int *idx);
+void drm_dev_exit(int idx);
 void drm_dev_unplug(struct drm_device *dev);
 
 /**
@@ -635,11 +637,16 @@ void drm_dev_unplug(struct drm_device *dev);
  * unplugged, these two functions guarantee that any store before calling
  * drm_dev_unplug() is visible to callers of this function after it completes
  */
-static inline int drm_dev_is_unplugged(struct drm_device *dev)
+static inline bool drm_dev_is_unplugged(struct drm_device *dev)
 {
-       int ret = atomic_read(&dev->unplugged);
-       smp_rmb();
-       return ret;
+       int idx;
+
+       if (drm_dev_enter(dev, &idx)) {
+               drm_dev_exit(idx);
+               return false;
+       }
+
+       return true;
 }
 
 
-- 
2.7.4


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